A metal-oxide-semiconductor (MOS) transistor is one of the most important devices in modern integrated circuits. A MOS transistor usually includes: a semiconductor substrate; a gate structure on the surface of the semiconductor substrate; a source in the semiconductor substrate on one side of the gate structure; and a drain in the semiconductor substrate on another side of the gate structure. The MOS transistor regulates currents in a channel under the gate structure by applying a voltage on the gate structure, to generate on/off signals.
As continuous developments of the semiconductor technology, the controlling ability of conventional planar MOS transistors on the channel currents becomes weaker, subsequently causing serious leakage current problems. To better adapt to the decreased size of the devices, focuses of the semiconductor technology have gradually changed from planar MOSFETs to more effective non-planar three-dimensional transistors, such as multi-gate devices including fin field effect transistors (FinFETs). A FinFET usually includes: fins protruding from a semiconductor substrate, gate structures covering a portion of top surfaces and sidewalls of the fins, a source in the fins on one side of each gate structure, and a drain in the fins on another side of each gate structure.
However, conventionally fabricated semiconductor devices including MOS transistors and FinFETs still have a poor performance. The disclosed devices and methods are directed to at least partially alleviate one or more problems set forth above and to solve other problems in the art.